Modified surfaces

ABSTRACT

The invention provides a method for producing a modified surface ( 5 ) comprising: patterning a surface ( 7 ) by forming thereon a porous molecular network ( 9 ) defined by non-covalent interactions between constituent molecules; and depositing in said porous network ( 9 ) and on said patterned surface ( 11 ) molecules ( 13 ) so as to form a self-assembled monolayer ( 15 ), wherein both said patterning and said depositing are effected by contact with liquids.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/003,213, filed Apr. 7, 2011, now allowed, which is a U.S. NationalStage application of International Patent Application No.PCT/GB2009/001701, filed Jul. 10, 2009, which claims the benefit ofpriority to GB Application No. 0812597.3, filed Jul. 10, 2008, each ofwhich is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to the processing of surfacesfunctionalised with a hybrid structure formed from (i) a molecularnetwork defined by non-covalent interactions between molecules and (ii)a self-assembled monolayer. Such hybrid structures may be processed bymodification and/or decoration, for example to yield surfaces havingelectronic, optical or other properties useful for a variety ofapplications.

BACKGROUND OF THE INVENTION

One of the central challenges in nanotechnology is the development offlexible and efficient methods for creating ordered structures withnanometre precision over an extended length scale. Supramolecularself-assembly on surfaces offers attractive features in this regard: itis a ‘bottom-up’ approach and thus allows simple and rapid creation ofsurface assemblies (De Feyter & De Schryver, Chem. Soc. Rev., 2003, 32,139-150; Barth, Annu. Rev. Phys. Chem., 2007, 58, 375-407) which arereadily tuned through the choice of molecular building blocks used andstabilized by hydrogen bonding (Theobald et al., Nature, 2003, 424,1029-1031; Kampschulte et al., J. Phys. Chem. B, 2005, 109,14074-14078), van der Waals interactions (Furukawa, Angew. Chem. Int.Ed., 2007, 46, 2831-2834), π-π bonding (Mena-Osteritz & Bauerle, Adv.Mater., 2006, 18, 447-451; Schenning & Meijer, Chem. Comm., 2005,3245-3258) or metal coordination (Diaz et al., J. Phys, Chem. B, 2001,105, 8746-8754; Stepanow, Nature Materials, 2004, 3, 229-233) betweenthe blocks. Assemblies in the form of two-dimensional open networks(Theobald et al., infra; Furukawa, infra; Mena-Osteritz & Bäuerle,infra; Stepanow, infra; Stöhr et al., Small, 2007, 3, 1336-1340;Spillmann et al., Adv. Mater., 2006, 18, 275-279; Lu et al., J. Phys.Chem. B, 2004, 108, 5161-5165; and Stepanow et al., Angew. Chem. Int.Ed., 2007, 46, 710-713) are particularly interesting for possibleapplications because well-defined pores can be used to preciselylocalize and confine guest entities such as molecules or clusters, whichcan add functionality to the supramolecular network,

Another widely used method for producing surface structures involvesself-assembled monolayers (SAMs) (Schreiber, J. Phys.: Condens. Matter,2004, 16, R881-R900), which have introduced unprecedented flexibility inproviding ability to tailor interfaces and generate patterned surfaces(Gooding et al., Electroanalysis, 2003, 15, 81-96; Love et al., Chem.Rev., 2005, 105, 1103-1170) But SAMs are part of a top-down technologylimited in terms of the spatial resolution that it can usually afford.

Additionally, skills and methodology are known in the art that allow thecreation of patterned organic layers on surfaces. These includemicrocontact printing, proximity printing, e-beam or ion beamlithography, photon-based patterning involving (photo) chemicalreactions, and scanning probe lithographies. As with existing SAMmethodologies, however, these additional top-down technologies arelikewise only able to provide limited spatial resolution and/or are slowserial processes.

J. A. Theobald et al. (in Nature 424, 1029 -1031(2003) and US2005/0214471 A1) describe the production of two-dimensional nanoscalenetworks on the surface of a substrate formed by deposition of twodifferent types of molecule. The formation of the network relies on thepreferential heteromolecular hydrogen-bonding between unlike moleculesover homomolecular interactions between like molecules. Resultant poresin the network are described as acting as containment vessels for guestmolecules. The networks are described as being prepared under ultra-highvacuum (UHV) conditions (base pressure approximately 5×10⁻¹¹ torr), amethod that is well-known to those skilled in the art.

WO 2008/006520 A2 describes a method for generating supramolecularrotary devices and a supramolecular rotary switch comprising providing atwo-dimensional layer of self-organising molecules on an unstructuredsurface followed by further deposition of additional self-organisingmolecules/or other functional molecules on the two-dimensional layer,the further deposited molecules accommodating in so-called functionalcentres of cells defined by the two-dimensional layer. The molecules aredescribed in this publication as having been vapour-deposited under UHVconditions on an atomically clean and flat Cu (111) surface.

Stepanow et al. (Chem. Commun., 2006, 2153-2155) describe thepreparation of so-called metallosupramolecular receptors that bind asingle or discrete number of cysteine, C₆₀ or diphenylalanine moleculesin which both the preparation of the two-dimensionalmetallosupramolecular receptors and the deposition of the guest speciesare undertaken under UHV conditions.

BRIEF SUMMARY OF THE INVENTION

The present invention arises from the recognition of advantagesachievable from combining non-covalent self-assembling porous networksand SAMs on surfaces, in particular the network and SAM are depositedfrom liquids. This combination provides a powerful and versatilefabrication platform distinct from the description in the prior art of“guest capture” within cavities of surface-deposited porous networks.The use of these two different surface modification strategies allowsthe creation of integrated network-SAM hybrid systems that can besufficiently robust to allow subsequent processing. In accordance withthis methodology the non-covalent self-assembling porous networksprovide nano-metre-scale precision and the SAM brings versatility to thesurface decoration.

It is particularly surprising, in the light of the prevalence in theprior art of deposition of porous networks from UHV environments, thatthe self-assembling (sometimes referred to herein as supramolecular orporous molecular) network and SAM components of the hybrid structure maybe deposited from liquid media (e.g. from solution). Such simple, and soadvantageous, deposition environments makes formation of the hybridstructures easier (obviating the need for conditions such as those underwhich UHV environments are achieved). Moreover, it broadens theversatility of the resultant structures and should enable widespread andflexible use of the invention.

Viewed from one aspect, therefore, the invention provides a method forproducing a modified surface (5) comprising:

-   -   (i) patterning a surface (7) by forming thereon a porous        molecular network (9) defined by non-covalent interactions        between constituent molecules; and    -   (ii) depositing in said porous network (9) and on said patterned        surface (11) molecules (13) so as to form a self-assembled        monolayer (15), wherein both said patterning and said depositing        are effected by contact with liquids.

Viewed from a second aspect there is provided a method for modifying ahybrid structure (5) comprised of (i) a surface (7) patterned with aporous molecular network (9) defined by non-covalent interactionsbetween constituent molecules in said porous network and (ii) aself-assembled monolayer (15) adsorbed on said patterned surface (11),said method comprising controllably chemically modifying the porousmolecular network (9) and/or the self-assembled monolayer (15).

By “controllably chemically modifying” is meant herein a modificationwhich may be carried out whereby to provide a modified product having apredicable degree of functionalization and/or modification relative tothe unmodified structure, which predicable degree of functionalizationand/or modification is not uncontrolled destruction of the hybridstructure in existence before the controlled chemical modification.

Viewed from a further aspect there is provided a product obtainableaccording to either the first or the second aspects of the invention.

Other aspects and embodiments of the invention will be apparent from thediscussion herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a)-(d) shows the structures of melamine (FIG. 1( a)); PTCDI(FIG. 1( b)); and bonding motif (FIG. 1( c) and (d)), FIG. 1( d) showingthe schematic diagram of network with unit cell indicated by dashedrhombus. FIG. 1( f) shows an alternative bonding motif. FIG. 1( e) showsa STM image of supramolecular network of melamine-perylenetetra-carboxylic di-amide (PTCDI) self-assembled on Au(111) recorded inambient. Dashed line A highlights a fault line. Circled areas B and Cmark a pore hosting a PTCDI molecule and a missing PTCDI molecule,respectively. The (7√3×7√3)R30° unit cell (D) corresponding to a 35 Åperiod of the honeycomb is also indicated. Inset shows Fouriertransform. Scale bar: 10 nm.

FIG. 2( a) shows schematically generation of a network-SAM hybridstructure (5) by a scheme of filling the cells (17) of thePTCDI-melamine network (9) by thiols (13) FIG. 2( b) shows structures ofthree different thiols (13) studied. FIG. 2( c) to (e) are STM images ofhybrid structures on Au(111)/mica in which the network (9) is filledwith adamantane thiol (ASH) (in FIG. 2( c)), dodecane thiol (C12SH) (inFIGS. 2( d)) and ω-(4′-methylbiphenyl-4-yl)propane thiol (BP3SH) (inFIG. 2( e)). Insets at lower left and upper right corners of STM imagesshow high resolution images and Fourier transforms, respectively. Scalebars: 20 nm for large scale images, 5 nm for insets.

FIG. 3( a) shows schematically the under potential deposition (UDP) ofCu (19) on Au(111) (7) modified by an ASH (13)-filled PTCDI-melaminenetwork (9), showing illustration of electrochemical Cu deposition inpores (17) of network at the thiol/Au interface (21). FIG. 3( b) and (c)show STM images of samples taken in ambient atmosphere after complete(b) and partial (c) Cu UPD. Scale bars: 20 nm. Arrows in FIG. 3( c) markisolated cells of Cu UPD. FIG. 3( d) shows height profile along theslanting shown line in FIG. 3( c), origin marked by “0”. Corrugationsare A=1.15 Å on UPD areas and B=0.5 Å on unaltered areas. Heightdifference between UPD and unaltered areas is S=1.3 Å.

FIG. 4 shows a cartoon of a generation of molecular hybrid structure (5)consisting of a supramolecular network (9) and pore-filling molecules(13) having headgroups (23) and tails (25) able to form SAM islands (15)in the pores (17). Molecules shown are an example of a network.

FIG. 5( a) shows, a scheme of possible modification pathways ofmolecular hybrid structures.

FIG. 5( b) shows (27) an example of modification of a hybrid structure.Metal is electrochemically deposited in the pores at the metal-SAMinterface (21) (reaction 1 in FIG. 5( a)).

FIG. 6( a) shows linear sweep voltammograms (LSVs) in an aqueouselectrolyte from three different structures (1-3) depicted schematicallyto the right of the LSVs.

FIG. 6( b) shows on the left, a scanning tunneling microscope image and,on the right, its Fourier transform image of structure 3 in FIG. 6( a).Pattern periodicity is 3.5 nm.

DETAILED DESCRIPTION OF THE INVENTION

Surfaces (7) upon which the porous non-covalently bonded network iscreated, whereby to pattern it, may be a surface of any convenientmaterial, referred to herein as substrate. The geometry of the substratemay also vary according to the requirements to which the hybridstructure is desired to be put. Thus the substrate may be a planarsubstrate with the hybrid structure coated upon one or both of itsprincipal faces; spherical structures with the hybrid structure coatedthereon; tubular structures with the hybrid structure coated on theinside and/or the outside of the tubes and other shapes and forms.

Conveniently, substrates (7) upon which it is desired to form the hybridstructures (5) of this invention may be thin layers typically of theorder of 100-500 nm, e.g. 150-300 nm. Bulky material (i.e. not thinlayers) may also be used, e.g. single crystals of substrate of ca. 0.5to 5 mm thick single crystals, or 0.5 to 1 mm thick silicon wafers. Thesubstrate (7) may be supported upon any convenient support formed ofeither the same material as or different material to that of thesubstrate. The substrate, when supported, or otherwise, need notnecessarily be as thick as 100 nm. Generally it is desirable to have asubstantially continuous surface of the substrate (i.e. one that is notgrainy on a nm length scale and this may be achieved at thickness ofless than 100 nm. The substrate may be, for example, made of glassessuch as silicates and borates, or conductors (e.g. metals) orsemi-conductors such as gold, silver, chromium, manganese, vanadium,tungsten, molybdenum, zirconium, titanium, platinum, aluminium, iron,steel, silicon, indium phosphide, gallium arsenide and alloys andmixtures thereof. The substrate can thus be a semiconductor such assilicon, gallium arsenide or titanium dioxide. The substrate may also bean insulator, e.g. silicon dioxide or aluminium oxide (Al₂O₃). Typicallyas described hereinafter, the substrate will generally not be chosen inisolation, the choice of substrate (7) influencing both the strength ofbinding of the porous molecular network (9) and self-assembledmonolayers (15) to be bound thereto.

Supported metallic layers may be conveniently prepared by a variety ofmeans as known by those skilled in the art such as by physical vapourdeposition methods (including thermal or electron beam evaporation),sputtering and electro deposition or electroless deposition.

Typically the metallic substrate is formed of gold, silver, copper,vanadium, platinum, palladium or nickel, more particularly gold.

The first step involved in the preparation of the hybrid structures (5)according to this invention is to adsorb to the surface of the substrate(7) a molecule or molecules capable of forming the desired extendedtwo-dimensional porous molecular network (9). This part of the method ofthe invention serves to provide an otherwise homogeneous surface with adesired pattern, in a predictable way. Patterning of surfaces may beachieved in this way by making use of non-covalent directionalinteractions between common or different molecules. Formation of such anetwork may be achieved in any convenient way as is known in the art.Thus, for example, formation of the network may be in accordance withthe description of US 2005/0214471 A1 (infra), typically forming thenetwork by contacting the substrate with the constituent molecules fromwhich the network is formed, whereby the two-dimensional network isprovided by depositing on the surface of the substrate a so-called“sub-mono-layer” of molecule A followed by a different molecule B.Stronger heteromolecular hydrogen-bonds between molecules A and B (asopposed to homomolecular bonding between like molecules) drive formationof the network. As is known in the art, the network can be formed byvirtue of hydrogen-bonds, van der Waals interaction or π-π bonding ormetal coordination between the different types of molecule. It will beappreciated that the (different) molecules that provide the network maybe provided in a single step or may each be present in two or moreseparate steps whereby they adsorb on the surface of the substrate toprovide the desired molecular network.

As will be understood, the precise identity of the components that formthe molecular network (9) is not crucial. The purpose of this part ofthe method is merely to provide a pre-determined (primarily by thenature of the molecules) and controllable deposition of a pattern ontosubstrate.

The nature of the molecules that form the network (9) may be determinedby a number of factors, for example the strength of interaction witheach other and the strength of interaction with the substrate (7). Ingeneral, any molecule or combination of molecules that exhibitsufficiently strong intermolecular interactions can be used. A featureof the present invention arises from a recognition that selection of theconstituent molecule(s) from which the network is formed in combinationwith selection of the substrate and selection of the molecules (13) thatform the SAM (15) give rise to hybrid structures that may be modified,in particular in liquid media.

An example of a pair of unlike molecules that can afford a desiredsurface patterning effect is perylene tetra-carboxylic di-imide (PTCDI)and 1,3,5-triazine-2,4,6-triamine (melamine). As depicted in FIG. 1( c),these molecules interact with each other via three hydrogen bonds. Thethree-fold and two-fold symmetry of melamine and PTCDI respectively,gives rise to a hexagonal network as shown schematically in FIG. 1( d).This bi-molecular network is, advantageously, particularly flexiblebecause the resultant pore size can be varied in a controllable fashionby using analogues of PTCDI (such as analogues of naphthalene,terrylene, and quaterrylene and coronene) and functionality by addingside-groups (e.g. fullerenes, aliphatic or aromatic moieties, aminoacids, metal-organic or organometallic moieties and others, such asfullerenes, aliphatic or aromatic moieties, amino acids, organometallicmoieties and others) to the aromatic rings in particular. Functionalgroups, such as carboxylic acids, aldehydes, ethers, amino groups,amides, alcohols and cyano groups, may be introduced in this way. Aswill be recognised by the skilled person, naphthalene-, terrylene-,quaterrylene- and coronene-based analogues of PTCDI are analogues ofPTCDI in that the naphthalene, terrylene, quaterrylene and coronene arefunctionalised, analogously to PTCDI, with two fused six-membered ringseach comprising the diradical —C(═O)N(H)C(═O)— joined to two carbonatoms of naphthalene, terrylene, quaterrylene and coronene.

As is evident from FIG. 1( c) in particular, melamine provides asuitable counterpart to PTCDI to provide for the formation of the threehydrogen bonds depicted. However, the skilled person will understandthat analogues of melamine could be used comprising the functional unitthat participates in the hydrogen-bonding network, i.e. (H₂N—C—N—C—NH₂).Such analogues of melamine are described herein as higher homologues ofmelamine.

Examples of higher homologues of melamine are known to and at thedisposal of those skilled in the art. They include, for example,4,4′,4″-(1,3,5-benzenetriyl)tris-2,6-pyridinediamine described byTheobald et al. (in Nature 424, 1029 -1031(2003) and US 2005/0214471 A1,both infra; see in particular compound 5 in FIG. 2 of US 2005/0214471A1):

as well as variants thereof, for example wherein the central benzenering is substituted with a different aromatic or other cyclic system, inparticular an aromatic system optionally allowing a different number(than 3), e.g. 2 or 4, in particular 4, of the 2,6-diaminopyridyl (orother H₂N—C—N—C—NH₂-containing) moieties to be attached. In suchhomologues, one or more spacer units, such as alkynylene,bis(alkynylene) or arylene diradicals, may be interposed between thecentral unit in the homologues (i.e. the benzene ring or other aromaticor other cyclic system) and pendant 2,6-diaminopyridyl (or otherH₂N—C—N—C—NH₂-containing) moieties. An example of such a variant iscompound 10 in FIG. 2 of US 2005/0214471 A1):

A further example of higher homologue of melamine is the compound

which comprises four of the docking H₂N—C—N—C—NH₂ units.

The syntheses of all such higher homologues of melamine as describedherein are well within the ability of those skilled in the art.

It is the presence of a plurality of hydrogen bond donors and/oracceptors that provides for the formation of a two-dimensional networkacross the surface of the substrate. Thus, for example, a moleculeexhibiting four of the functional units in melamine (H₂NC—N—C—NH₂), andexhibiting a four-fold axis of symmetry, will enable the construction ofrectangular, for example, square pores, instead of the hexagonal poresresultant from the use of PTCDI and melamine. The ability to tailor thepore size by the judicious selection of the constituent molecules fromwhich the molecular network is defined is an advantageous feature ofthis invention.

From the foregoing, therefore, it will be appreciated that the specifichydrogen-bonding motif depicted in FIG. 1( c) is but a single example.Others are possible, such as that depicted schematically in FIG. 1( f)in which each of the arbitrarily positioned substituents R¹ and R² serveto indicate that an additional organic moiety, for example but notlimited to benzene-, perylene-, coronene- and phthalocynine-containingmoieties could be present. Such moieties may then participate inaddition hydrogen-bonding interaction (for example of the type indicatedin FIG. 1( c)), the number, position and nature of such substituentsbeing selected on the basis of the nature of the porous molecularnetwork (9) desired. In principle, any hydrogen-bonded motif may be usedto construct the molecular network according to the present invention,with the hydrogen bonding motif possibly being made up of amino, imino,keto, hydroxyl or carboxylic acid groups, amongst other groups evidentto the skilled person.

The advantageous features (in particular) of the PTCDI and melaminemolecules and analogues thereof are the presence of both aromaticmoieties capable of interacting strongly with metallic substrate, suchas gold, as well as moieties that allow a strong interaction betweenmolecules, whereby to form the network. Typically these moieties giverise to a strong hydrogen-bonding network resulting from a plurality ofhydrogen bonds between molecules although other interactions may alsoarise between constituent molecules.

As an alternative to the use of melamine and PTCDI, or analoguesthereof, to provide the desired surface patterning effect, moleculessuch as porphyrins, coronenes and phthalocyanines, and derivativesthereof may be used to form planar molecular network structures whenfunctionalised as described in WO 2008/006520 (infra). As is describedin this publication, network structures may be formed in this way from asingle type of molecule such as a porphyrin derivative. Other molecularbuilding blocks, such as dehydro-benzo[12]annulene derivatives, may beused to construct the two-dimensional molecular network as described,for example, by S. Furukawa et al. (Angew. Chem. Int. Ed. 2007, 46,2831-2834). Other means of forming molecular networks by adsorption ontosubstrates, and appropriate compounds therefore, are known to and at thedisposal of those skilled in the art.

Typically, in the prior art, as alluded to hereinbefore, construction ofthe molecular network on substrates is achieved in UHV environment. Thisrestricts assembly to molecules susceptible to sublimation and can makeadditional processing of the resultant network difficult. Incontradistinction, we have found that it is generally possible thoughnot necessary according to all aspects of this invention to manufacturethe porous network by a liquid-based fabrication strategy, as well as bythe UHV processing practiced hitherto. It is the recognition of thispossibility in particular that spurred us to investigate and achieve thesubsequent addition of SAMs onto the patterned substrate provided bydeposition of the porous two-dimensional network.

As we describe below, in the experimental section, the liquid-baseddeposition of PTCDI/melamine network onto gold has been achieved from asolution of dimethylformamide (DMF). It will be appreciated that anyconvenient liquid for the component(s) serving to give rise to theporous network may be used in place of DMF.

Thus other organic liquids such as dimethylsulfoxide (DMSO), toluene andothers, or water and water-containing liquids, where appropriate, may beused in place of DMF. In particular embodiments of this inventiondeposition onto the substrate of the molecules that afford the networkis achieved from solution. However deposition may also be achieved bydeposition from dispersions, suspensions or emulsions of the molecules.

We have found that a particular advantage of the liquid-based depositionis that it is possible to provide an extended two-dimensional porousmolecular network, in contrast to the 50% coverage reported by J.A.Theobald et al. (infra). Indeed, the network structure overall is veryregular. An STM of a melamine-PTCDI network is shown in FIG. 1( e),which shows that there are no major discontinuities but someimperfections are discernible. The first one, highlighted by the dashedline A in FIG. 1( e), is a fault line with neighbouring hexagons meetingat a vertex instead of sharing an edge. The second one is an additionalPTCDI molecule trapped in a pore (marked by ellipse B). A third one is amissing PTCDI molecule (ellipse C in FIG. 1( e)), thus, joining twoadjacent cells. However, these are exceptions to the overall pattern ofthe surface, rather than the coverage of the surface by the pattern, thecoverage being essentially continuous.

The liquid-based preparation makes the network a readily accessibletemplate, but the scope for further modification and use depend on itsstability under the conditions of subsequent processing, in particularmodification with SAMs. We have found that the supramolecular networkstructure of the invention exhibits sufficient stability to act as atemplate for the adsorption onto the pattern defined by it whereby toprovide the hybrid structure.

By SAM herein is meant a layer that self-assembles on a surface. Theassembly is a monolayer because it is formed of a plurality of (usuallyorganic) molecules that have a particular affinity, in a portion oftheir structure commonly referred to as a headgroup, for the surface onwhich they assemble. It is the interaction of the headgroup with thesurface that leads to formation of the monolayer. The skilled person isfamiliar with which types of headgroup bind to any particular surface,for example which headgroup best binds to a specific metal and referencein this regard is made to the extensive discussion of the use innanotechnology of self-assembling monolayers (of thiolates inparticular) on metals by J. C. Love et al. (Chem. Rev., 2005, 105,1103-1169) and in particular the discussion therein of exemplarycombinations of headgroups in substrate used in forming SAMs on metals,oxides and semiconductors (see Table 1).

The headgroups may conveniently be thiols, the molecules providing theSAMs typically being organothiols, i.e. of formula R—SH where R is anorganic moiety. For details see J. C. Love et al. (infra). As is knownin the art, disulfides can also be used to provide SAMs as well asselenium containing analogues of thiols and disulfides. Other headgroupsare known to those skilled in the art and include phosphonates,carboxylic acids, silanes and other functional groups, e.g. phosphatesand sulfonic acids, capable of forming a covalent or ionic bond, e.g. acovalent bond, to the substrate such as e.g. OH or unsaturated C—Cbonds. In order to form the SAM layer onto the patterned surface, themolecules that form the SAM may be dissolved, emulsified or dispersed inany convenient liquid whereby to form an appropriate solution, emulsionor dispersion. Typically, the molecules will be dissolved whereby toform solutions, e.g. of concentrations between about 1 nM and 1 M, e.g.between about 1 μM and 100 mM, more particularly from about 0.01 to 10mM. The appropriate liquid will be dependent upon the molecule concernedand can be selected by the skilled person. For example, thiols may beappropriately dissolved in organic solvents, e.g. alkanols, for examplemethanol, ethanol, isopropropanol, or mixtures thereof. Contact timestypically vary between about 1 second and 24 hours. After formation theresultant hybrid structure may be rinsed, generally with more of thesame solvent in which the molecules forming the solution etc. weredissolved, followed by drying. Drying may be achieved by air-drying,blowing air or inert gas over the structures or by other ways, forexample drying in an oven at a suitable temperature, optionally underreduced pressure.

The hybrid structures of the invention are stable in a liquidenvironment in which they may be formed and can be processed further.For example, the SAM portion of the hybrid structure may be modified soas to tailor the surface functionality displayed by the SAM to providesurfaces useful for various applications. Reference is made to thereview of J. C. Love at al. (infra) for various methodologies known inthe art for modifying SAMs.

The hybrid structures described herein may be prepared by liquid-basedtechniques described herein. Alternatively they may be prepared in otherways, for example under UHV conditions. Regardless of how they may beprepared, however, the hybrid structure may be modified to affordspecifically modified substrates. This is achieved by controlledchemical modification of the hybrid structures described herein by whichis meant a process whereby the hybrid structure is altered in anon-destructive manner and in which the modification is specific to aparticular portion of the hybrid structure. For example, it is possibleto effect electrochemical metal deposition in a potential range where amonolayer of metal may be deposited known as underpotential deposition(UPD), (effected at a potential more positive compared to one where bulkmetal deposits) resulting in the intercalation of the metal (19) at theSAM-substrate interface (21) (shown pictorially in step 1 of FIG. 5).For example, if copper is deposited on a SAM/Au sample the R—S—Aubonding is replaced by R—S—Cu—Au. The consequence of this intercalatedis that the bonding of thiol (or other molecules giving rise to a SAM)can become stronger. For example, if copper or silver are intercalatedbetween a SAM and a gold substrate, the thus-modified interfaceincreases stability of the SAM-decorated substrate.

Whilst UPD on gold-supported thiols is typically practised with copperor silver intercalation, intercalation of other metals or non-metals ispossible where the bonding of the thiol or other molecule making up theSAM to the substrate is weaker than its bonding via an intercalatedmaterial.

A particular advantage of the present invention is that the geometry ofthe hybrid structure allows targeted chemical modification of either theSAM or the macromolecular network through the pattern of which the SAMis adsorbed to the underlying substrate. In other words the patterningof the molecular network into which the SAM is deposited means that theresultant SAM formed is also patterned. An example of this is the UPD ofmetals described hereinabove. Whilst UPD of metals on metal-supportedself-assembled monolayers is known, it has hitherto been a problem toconfine this deposition laterally (C Silen and M Buck, J. Phys, Chem.,2008, 3881-3890), because SAMs have typically been present over thewhole surface providing essentially homogenous extended regions of SAM.With respect to nanotechnology, i.e. the generation of nanometre (i.e.typically in which one dimension is less than 100 nm) scaled patterns,this is a serious limitation as the inability control the pattern ofdeposited metal reaches its resolution. This may be contrasted with theSAM present in the hybrid structure according to this invention sincethere is strict demarcation between “islands” of SAM provided by theunderlying network in the pores of the network within which the SAMmolecules sit. Thus, when subjected to UPD, the metal or other materialintercalated into, for example, the Au-S bond, is confined to thoseareas of the substrate on which the SAM is present.

Moreover, not only does the UPD on the SAM-network hybrid system avoidintercalation over extended areas of SAM as in the prior art, becausethe network acts as a barrier to the diffusion of intercalated metal,UPD proceeds very quickly as a consequence of the gaps between the SAMislands, Furthermore, UPD of an extended layer of SAM previously wouldcorrelate with the defects in the epitaxiality of the SAM deposition onthe substrate. In contrast, the regular structure of the hybridstructure of this invention may be considered to be a series ofwell-defined defects (i.e. defined by the gaps between the SAM islands),which greatly facilitate UPD making it more controllable.

Separately, and as an alternative to or in addition to any intercalationchemistry conducted at the interface of the SAM and the substrate, it ispossible to effect chemistry on the “tails” of the molecules forming theSAM whereby to, for example, functionalise these portions of theSAM-forming molecules. Thus, nanometre-sized objects (29) may beintroduced to sit on top of the SAM islands, for example by introducingchemical functionality into the tails of the molecules that provide theSAM. Different ways to introduce such objects (29) are depictedschematically as the product of steps 2 shown in FIG. 5. An example isthe attachment of oligonucleotide (such as DNA or RNA) or proteins.Alternatively, functionality may be present in the tails (25) allowingcoordination of metal ions to the top of the SAM island. Electrochemicalreduction of the ions to atomic metal may be effected which resultantmetal atoms can aggregate to nanometre-sized clusters (29). Suchclusters can be advantageous in various electronic applications such asthose based upon quantum dot technology (Oncel et al, J. Chem. Phys.,2005 123, 044703/1-4, Shekhah et al., PCCP 2006, 8, 3375-3378).

With regard to modification of the macromolecular network (5), this isdepicted in steps 3 of FIG. 5. One way in which the network (9) may bedisplaced, if this is desired, is by its substitution, i.e. replacement,with one or more type(s) of molecules (14) that may form self-assembledmonolayers. These second or subsequent molecules (14) need not be (andadvantageously are not) the same molecules as those (13) which providethe SAM islands (15) occupying the pores of the macromolecular network(9) nor even have the same headgroups (23) as those molecules (13). Ifthe macromolecular network is displaced in this way, the resultantmodified substrate (3-2, 3-4, 4) comprises a substrate (7) decoratedwith the original SAM (15), in which the gaps between the SAM islands ofthe initial hybrid structure are filled with the species (14) thatserved to displace the network molecules. Since the original networkdefines where the additional species (14) are located, such displacementmethod allows for the generation of very exactly patterned SAM-decoratedsurface (see structures 3-2 and 3-4 (and 4) of FIG. 5) since the networkserves as the pattern for the initial layer of SAMs on an extended area,its displacement may be regarded as a massively parallel yet easilyachievable process. This may be contrasted with the process in the priorart in which the generation of patterned SAMs at this degree ofresolution (i.e. less than 100 nm and in particular less than 10 nm)requires cumbersome, time-consuming serial techniques (such as scanningprobe microscopy) which are both harder to control and not applicable toextended areas and non-flat substrate geometries.

As an alternative to displacement with a species that is also a type ofmolecule susceptible to the formation of a SAM, network displacement maybe effected by electrochemical deposition of metal or other metalsleading to the structures 3-1 and 3-3 shown in FIG. 5.

Displacement of the network can either take place on a hybrid structurethat has not been subject to intercalation by, for example, UPD wherebyit has stabilised the substrate-SAM molecule interactions (see routes 3a-c of FIG. 5) or on a hybrid structure on which the SAM islands havebeen stabilised by UPD metal intercalation (see routes 3 a′-c′ of FIG.5). Also, a displacement of the network may be by a “direct” process asdepicted in routes 3 c and 3 c′ of FIG. 5 or by sequential displacementshown by routes 3 a & b and 3 a′ & b′ of FIG. 5.

Further modifications of the hybrid structures are shown by the arrowsleading to structure 4 in FIG. 5. The arrow leading from structure 3-1to structure 4 depicts the adsorption of molecules onto metal depositedin step 3 b′; the arrow pointing from structure 3-2 may afford the samestructure but as a result of, for example, UPD of a metal beneath asecond type of SAM molecule introduced in direct substitution route 3c′.

Whether the molecules that form the SAM in the SAM-modified structure ofthe macromolecular patterned substrate serve as active sites for preciselocalisation of species through chemical interactions, or whether theyare used to block such interactions, whereby for example to directfurther molecules to the network molecules themselves (since these arenot covered by the molecules that form the SAM), the hybrid system canprovide control on a scale and at a precision not readily achievableotherwise. By accessing the hybrid systems through exclusivelyliquid-based processing in particular, according to certain embodiments,facilitates a wide range of fundamental studies into confinednanometre-sized geometries can influence phenomena as diverse aselectrochemistry, tribology or wetting.

An example of the further proccessability of the hybrid structure ofthis invention is the electrochemical deposition of copper in the underpotential deposition (UPD) region as described below.

The invention may be further understood with reference to the followingnon-limiting examples:

General

FIG. 1( e) is despeckled, all other STM images are presented asacquired.

EXAMPLE 1 Formation of Hybrid Structure

PTCDI (Alfa Aesar, 98+%) and melamine (Sigma-Aldrich, 99.9%) were usedwithout further purification. The PTCDI/melamine mixture used for theexperiments was prepared from saturated solutions of PTCDI and melaminein dimethyl formamide (DMF) which were diluted, typically by factors of25 and 4 for PTCDI and melamine, respectively. Au/mica substrates (300nm Au, G. Albert PVD) were flame annealed prior to immersion into thePTCDI/melamine solution, Immersion times for network formation were upto 3 min at temperatures varying from 325 and 400 K with 1 min and 371 Kas a typical combination of parameters. After removal from solutionsamples were blown dry in a stream of nitrogen or argon. The STM imageof the resultant network in FIG. 1 e reveals the honeycomb arrangementof the PTCDI molecules, which are the moieties resolved on this scale.The period of the honeycomb is 35 Å which corresponds to a (7√3×7√3)R30°unit cell (Perdigao, L. M. A. et al J. Phys. Chem. B 2006, 110,12539-12542 (2006)). In contrast to the 50% coverage observed in anearlier UHV experiment (Theobald et al., Nature, 2003, infra), we findthat the network forms over extended areas.

A precise estimation of network stability under relevant conditions isnot possible, due to the lack of precise data for the network, inparticular for the adsorption energies of PTCDI and melamine. But we canuse the hydrogen bond energy per synthon (values range from 70 kJ/mol(Weber et al., Phys. Rev. Lett. 2008, 100, 156101/1-4) and 90 kJ/mol(Aakeroy, C. B. & Seddon, Chem. Soc. Rev. 1993, 22, 397-407) tocalculate total network binding energies of 140-180 kJ/mol and 210-270kJ/mol per PTCDI or melamine molecule, respectively. The adsorptionenergies of PTCDI and melamine are taken to be similar to those of otheraromatic hydrocarbons (Baldacchini, C., Mariani, C. & Betti, M. G. J.Chem. Phys. 2006, 124, 154702/1-7; Bilic, A., Reimers, J. R., Hush, N.S., Hoft, R. C. & Ford, M. J. J. Chem. Theory Comput. 2006, 2,1093-1105), which range from 50 to 200 kJ/mol. With this approach, weestimate the binding energy of a network molecule to fall in the rangeof 200-470 kJ/mol, which is higher than the 160-200 kJ/mol of an Au—Sbond (Schreiber, F. J. Phys.: Condens. Matter 2004, 16, R881-R900; Love,J. C., Estroff, L. A., Kriebel, J. K., Nuzzo, R. G. & Whitesides, G. M.Chem. Rev. 2005, 105, 1103-1170). But considering that more than onethiol molecule can be adsorbed in the area occupied by PTCDI andmelamine, we conclude that thiol adsorption can energetically match thenetwork.

To investigate to what extent thiols can be adsorbed into the network wechose three types of molecules (see FIG. 2 b)) which differ in thestability of the respective SAMs. One is small and rigid and has ratherweak intermolecular interactions (adamantane thiol, ASH) (Dameron, A.A., Charles, L. F. & Weiss, P. S. J. Am. Chem, Soc., 2005, 127, 8697-8704); the other two exhibit more pronounced intermolecularinteractions, one of these consisting of a rigid aromatic moietycombined with an aliphatic spacer (ω-(4-methylbiphenyl-4-yl)propanethiol, BP3SH) and the other of a flexible alkane chain (dodecane thiol,C12SH).

For thiol adsorption experiments network/Au/mica samples were immersedin a 1 mM solution of the respective thiol (adamantane thiol (ASH):Sigma-Aldrich, 99.9%, dodecane thiol (C12SH) Sigma-Aldrich, 98+%,ω-(4′-methylbiphenyl-4-yl)propane thiol (BP3SH), synthesis see ref. 43in J. Phys. Chem. C, 2008, 112, 3881) in ethanol at room temperature.Immersion times were varied between 3 s and 24 h. After immersion,samples were thoroughly rinsed with ethanol and blown dry with N2.

Large-scale STM images of the resultant structures (FIG. 2( c), (d),(e)) show that the network acts as template for all three types ofmolecules, with high-resolution images and Fourier transforms (seeinsets) confirming that in all cases the hexagonal pattern is wellmaintained after thiol adsorption. In contrast to the empty networkwhere the molecules appear as protrusions (FIG. 1), filling the networkpores inverts the height contrast so that the presence of the network isreflected by the appearance of hexagonal grooves. It is worth notingthat due to the rigidity of adamantane thiol it was even possible toachieve molecular resolution (inset of FIG. 2( c)).

FIG. 2 demonstrates that the supramolecular network serves as a generaltemplate for a range of thiol molecules that form SAMs differingsubstantially in structure, intermolecular interactions and stability.But we note that the details of the preparation protocol relate to theSAM molecule used, and reflect the above estimated similarity of SAMsand network with respect to their energetics. In the case of adamantanethiol, which is known to form SAMs that are not very stable (compared toSAMs formed from e.g. alkane thiols), immersion time is not critical:the pores of the network are filled within seconds, and the networkitself is perfectly stable against displacement by ASH. In contrast, inthe case of the other two molecules, prolonged exposure of the networkto a solution of the respective thiol molecules ultimately results inthe displacement of the network and formation of a uniform SAM. However,there is a pronounced difference between the rate at which the pores arefilled and the rate at which the network is displaced, so that selectiveadsorption in the pores while maintaining the network structure can bekinetically controlled as evidenced by FIG. 2.

EXAMPLE 2 Electrochemical (UPD) Deposition of Copper onto HybridStructure

Partial Cu UPD was achieved in 50 mM CuSO₄/0.5 μM H₂SO₄ (aqueous) bysetting the sample potential at +100 mV versus Cu/Cu²⁺ for 10 sec in aPTFE electrochemical cell. The sample was then rinsed with deionised H₂Oand blown dry with N₂. Complete Cu UPD coverage was achieved byrepeating the same procedure once.

The experiment shown schematically in FIG. 3( a) involves a sample witha SAM/network hybrid structure mounted in an electrochemical cellcontaining Cu²⁺ ions. A potential in the UPD region of Cu (Le., positiveof the Nernst potential) is then applied, which causes insertion of amonolayer of Cu between the Au substrate and the thiol molecules(Silien, C. & Buck, M., J. Phys. Chem. C 112, 3881-3890(2008)). The Cuinsertion renders the thiol/substrate bond more stable and could be usedfor further patterning (Oyamatsu, D., Kanemoto, H., Kuwabata, S. &Yoneyama, H., J. Electroanal. Chem. 497, 97-105 (2001)). Afterdeposition, the sample was removed from the cell and investigated by STMin ambient environment, with the image (FIG. 3( b)) revealing that thepattern of the hybrid structure is preserved.

To probe the insertion of Cu, experiments were performed with adeposition time chosen such that Cu UPD has not yet occurredhomogeneously across the whole sample. In the STM image (FIG. 3( c)),the hexagonal structure is discernible in both the unaltered and the UPDareas. In contrast, the corresponding height profile (FIG. 3( d))reveals an increase in height S due to Cu UPD. A most notable feature ofFIG. 3( d) is the difference in the corrugation between the UPD and theunaltered area, respectively. On the UPD part the corrugation A issignificantly larger compared to the corrugation B of the unalteredarea. This strongly suggests that Cu is only inserted between thiol andsubstrate and not between network and substrate as illustrated in FIG.3( a), i.e., the network acts as a diffusion barrier. Thisinterpretation is corroborated by the appearance of isolated UPD islands(marked by arrows in FIG. 3( c)) where just one cell is filled. Thesuppression of Cu diffusion at the interface by the network makes thehybrid system very different from a uniform SAM where Cu UPD cannot beconfined due to the lack of such a diffusion barrier (Silien & Buck,infra). We also note that compared to densely packed SAMs, intercalationof the Cu ions at the thiol-substrate interface is greatly facilitatedand faster for the hybrid system due to the more open structure.Overall, the hybrid system renders UPD on the nanometre scale much morecontrollable than when using a SAM without network.

Samples were characterized under ambient by scanning tunnelingmicroscopy (STM) using a PicoPlus STM (Molecular Imaging). Bias andcurrents were typically in the range of 250- 800 mV (tip positive) and5-80 pA.

EXAMPLE 3 Displacement of Molecular Network of a Hybrid Structure byAdditional SAM-Forming Molecules

Hybrid structures were prepared as described in Example 1 above in whichpores within a network formed from PTCDI and melamine formed on 300 nmthick epitaxial gold film evaporated onto mica and in which the pores ofthe network were filled either ASH or an aliphatic spacer(ω-(4′-methylbiphenyl-4-yl)ethane thiol, BP2). The resultant hybridstructures were then modified by UPD deposition of copper between thethiol moieties of the ASH and BP2 SAMs and the underlying gold/micasubstrate as described in Example 2. The thus-prepared hybrid structureswere then subject to displacement of the PTCDI/melamine molecularnetwork by thiols. This displacement was demonstrated using 0.5 nMsolution of ASH in a mixture of ethanol/water (1:1) and H₂SO₄ (0.5 μM).The copper-modified hybrid structures were exposed to this solutionunder potential control (minus 0.6 V vs Ag/AgCl reference electrode)typically for 10 minutes, which resulted in replacement of thePTCDI/melamine molecular network by ASH. In this way, it is possible toprepare a SAM patterned on the sub-5 nm scale that comprises twodifferent types of SAM-forming molecules (e.g. BP2 and ASH in thepresent example).

In this example the SAM-forming molecule displacing the PTCDI/melaminenetwork (i.e. ASH) is different from the molecules forming the existingSAM within the pores of the network (i.e. BP2). This need notnecessarily be the case, however: molecules of BP2 could likewisedisplace the network molecule to provide a surface decorated only with asurface comprising BP2 molecules, Advantageously, however, as describedhereinbefore, the network, if displaced, may be usefully displaced witha different type of SAM-forming molecule (in particular with chemicalfunctionality, advantageously at the tail ends of the molecules)allowing patterning of the underlying surface on the sub-5 nm scale.

This example is thus an embodiment of step 3 c′ in FIG. 5( a) whereby toprovide structures of the type 3-2 depicted. Evidence that structures ofthis type were formed was achieved by way of conducting linear sweepvoltammograms (LSVs) and by scanning tunneling microscopy. The resultsof these experiments are depicted in FIG. 6( a) and FIG. 6( b).

FIG. 6( a) shows the electrochemical characterisation of the desiredsurface functionalised with the two different types of SAM-formingmolecule, i.e. of structure 3-2 as depicted in FIG. 5( a). The LSV ofthis structure in an aqueous electrolyte is indicated with the number 3in the LSVs shown on the left hand side of FIG. 6( a). LSVs 1 and 2correspond to the structures 1 and 2 depicted on the right hand side ofFIG. 6( a) in which 1 is a controlled uniform SAM of ASH; and 2 is acopper-modified PTCDI/melamine network/BP2 SAM hybrid structuredescribed above before it has reacted with ASH to provide structure 3.

The LSVs were recorded in a 0.25 nM solution of KOH in ethanol/water(1:1) at a scan rate of 10 mV/s. A Teflon cell purged with nitrogen wasused with platinum wires as pseudo reference and counter electrode,respectively.

The peaks seen in the LSVs demonstrate the desorption of the ASH SAMmolecules with no peak detected in the LSV of structure 2 indicatingthat SAM within the pore is stable in the potential range shown. Afterreplacement of the PTCDI/melamine network by the type of SAM moleculeshown in structure 1 (i.e. ASH) a peak is clearly detected in the sameposition of the LSV of structure 3 corresponding to the peak of “pure”ASH in the LSV of the structure 1. This indicates that the replacementof the network structure with ASH was successful.

FIG. 6( b) shows, on the left hand side, a scanning tunneling microscopeimage and, on the right, its Fourier transform of structure 3 indicatedin FIG. 6( a) and was recorded in ambient using a Pt/Ir tip (80:20) andtunneling parameters of 2-5 pA±(0.2-0.5) V. The STM image evidence isthat the pattern imposed by the network is maintained throughout thereplacement process. The periodicity of the pattern is 3.5 nm.

It is claimed:
 1. A method for modifying a hybrid structure, the hybridstructure comprising: (i) a surface patterned with a two-dimensional,non-covalent self-assembled porous molecular network defined bynon-covalent interactions between constituent molecules in said porousnetwork; and (ii) a self-assembled monolayer adsorbed on the patternedsurface, the self-assembled monolayer formed of molecules eachcomprising a headgroup having a particular affinity for the patternedsurface, said method comprising controllably chemically modifying theporous molecular network and/or the self-assembled monolayer.
 2. Themethod of claim 1, wherein the hybrid structure is produced by a methodcomprising: (i) patterning a surface by forming thereon the porousmolecular network; and (ii) depositing in the porous network and on thepatterned surface the molecules so as to form the self-assembledmonolayer, wherein both the constituent molecules forming the porousmolecular network and the molecules forming the self-assembled monolayerare deposited on the surface from liquids.
 3. The method of claim 1wherein the controlled chemical modification comprises underpotentialdeposition, resulting in the intercalation of metal at the SAM-surfaceinterface.
 4. The method of claim 1 wherein the controlled chemicalmodification comprises functionalising the tails of molecules thatcomprise the self-assembled monolayer with nanometre-sized objects, suchas oligonucleotides, proteins or clusters of aggregated metal atoms. 5.The method of claim 1 wherein the controlled chemical modificationcomprises displacement of the porous molecular network.
 6. The method ofclaim 5 wherein the controlled chemical modification comprisesreplacement of the porous molecular network by further molecules to forma self-assembled monolayer.
 7. The method of claim 6 wherein thedisplacement of the porous molecular network is effected by moleculesdifferent to those from which the self-assembled monolayer is formed. 8.The method of claim 2 wherein the patterning is achieved by depositionof the molecules that afford the network onto the surface from asolution, a dispersion, a suspension or an emulsion of the molecules. 9.The method of claim 1 wherein the surface is of a substrate formed ofthe group consisting of gold, silver, chromium, manganese, vanadium,tungsten, molybdenum, zirconium, titanium, palladium, platinum,aluminium, iron, steel, silicon, indium phosphide, gallium arsenide, andalloys comprising one or more of the foregoing, and mixtures thereof.10. The method of claim 1 wherein the surface is of a substrate formedof a metal.
 11. The method of claim 10 wherein the metal is selectedfrom the group consisting of gold, silver, copper, vanadium, platinum,palladium and nickel.
 12. The method of claim 1 wherein the porousmolecular network (9) is defined by non-covalent interactions betweencommon or different types of constituent molecules.
 13. The method ofclaim 12 wherein the constituent molecules comprise two or moredifferent types of molecules.
 14. The method of claim 13 wherein theconstituent molecules comprise two or more different molecules selectedfrom the group consisting of (i) melamine and higher homologs ofmelamine; and (ii) perylene tetra-carboxylic di-imide (PTCDI) and PTCDIanalogs of naphthalene, terrylene, quaterrylene and coronene, which areoptionally functionalised with one or more moieties selected from thegroup consisting of fullerenes, aliphatic or aromatic moieties,metal-organic or organometallic moieties, carboxylic acids, aldehydes,ethers amino groups, amides, alcohols and cyano groups.
 15. The methodof claim 14 wherein the constituent molecules comprise melamine andPTCDI.
 16. The method of claim 12 wherein the porous molecular networkis defined by non-covalent interactions between a single type ofconstituent molecule selected from the group consisting of porphyrins,coronenes, phthalocyanines and derivatives thereof, anddehydro-benzo[12]annulene derivatives.
 17. The method of claim 1 whereinthe self-assembled monolayer is formed from molecules selected from thegroup consisting of thiols, sulfides, or selenium-containing analogsthereof, phosphonates, phosphates, carboxylic acids and sulfonic acids.18. The method of claim 1 wherein the self-assembled monolayer is formedfrom molecules deposited from a solution, emulsion or dispersion. 19.The method of claim 2 wherein the patterning is effected by contact ofthe surface with the constituent molecules that form the porousmolecular network in a single step.
 20. A product obtained by a methodof claim
 1. 21. A product obtained by a method for producing a modifiedsurface comprising: (i) patterning a surface by forming thereon theporous molecular network; and (ii) depositing in the porous network andon the patterned surface the molecules so as to form the self-assembledmonolayer, wherein both the constituent molecules forming the porousmolecular network and the molecules forming the self-assembled monolayerare deposited in surface from liquids.